As one scheme of detecting the focus state of an imaging lens, Japanese Patent Laid-Open No. 58-24105 (to be referred to as “patent reference 1” hereinafter) discloses an apparatus that performs focus detection of the pupil division scheme using a two-dimensional sensor having pixels in each of which a microlens is formed. The apparatus in patent reference 1 is configured such that a photo-electric conversion unit in each pixel which constitutes the sensor is divided into a plurality of photo-electric conversion units, which receive light beams in different areas on the pupil of the imaging lens via the microlens.
Also, Japanese Patent No. 2959142 (to be referred to as “patent reference 2” hereinafter) discloses a solid-state image sensor, which serves both as an image sensor and a focus detector and has a two-dimensional array of pixels in which the relative positions between microlenses and photo-electric conversion units are shifted from each other. The solid-state image sensor described in patent reference 2 detects the focus state of an imaging lens based on images generated by pixel arrays having different directions of relative shift between microlenses and photo-electric conversion units. On the other hand, this sensor captures a normal image by adding pixels having different directions of relative shift between microlenses and photo-electric conversion units.
Also, the applicant of the present invention filed Japanese Patent Laid-Open No. 2005-106994 (to be referred to as “patent reference 3” hereinafter) that discloses a solid-state image sensor which performs focus detection of the pupil division scheme using a CMOS image sensor (solid-state image sensor) adopted in a digital still camera. The solid-state image sensor in patent reference 3 is configured such that some of a large number of pixels, which constitute the solid-state image sensor, each include a photo-electric conversion unit divided into two sections so as to detect the focus state of an imaging lens. The photo-electric conversion unit is configured to receive light in a predetermined area on the pupil of the imaging lens via the microlens.
FIG. 20 is an explanatory view of the distribution of light received by a pixel that performs focus detection and is located at the center of the solid-state image sensor disclosed in patent reference 3. FIG. 20 shows areas on the pupil of the imaging lens, in which the two divided photo-electric conversion units can respectively receive light. Crosshatched areas within a circle in FIG. 20 indicate the exit pupil of the imaging lens, and non-crosshatched areas Sα and Sβ indicate areas in which the two divided photo-electric conversion units can respectively receive light. The areas Sα and Sβ are normally set symmetric about the optical axis (the intersection between the x- and y-axes in FIG. 20) of the imaging lens.
The camera performs correlation calculation between an image generated by a light beam transmitted through the area Su on the pupil of the imaging lens and that generated by a light beam transmitted through the area Sβ on that pupil, thereby detecting the focus state of the imaging lens. Japanese Patent Laid-Open No. 5-127074 (to be referred to as “patent reference 4” hereinafter) discloses a method of focus detection by correlation calculation between images generated by light beams transmitted through different pupil areas of the imaging lens. Patent reference 4 also discloses a technique of deforming a specific filter stored in the camera in accordance with the aperture ratio, the exit pupil position, and the amount of image shift, adapting the deformed filter to an object image, and detecting the image forming state.
The focus state can be detected not only for an object positioned at the center of the image capturing window but also for an object normally positioned in the periphery of the image capturing window. However, an eclipse of a light beam is often caused by, for example, the frame of the imaging lens in the periphery of the image capturing window. When this occurs, the areas Sα and Sβ on the pupil of the imaging lens become asymmetric. Therefore, an image generated by a light beam transmitted through the area Sα on the pupil of the imaging lens has low degree of matching with that generated by a light beam transmitted through the area Sβ on that pupil in that case. As a result, the techniques disclosed in patent references 1 to 3 have a disadvantage that high-accuracy focus detection is impossible even when correlation calculation is performed based on an image generated by a light beam transmitted through the area Sα on the pupil of the imaging lens and an image generated by a light beam transmitted through the area Sβ on that pupil.
Also, the technique disclosed in patent reference 4 has a disadvantage that image correction corresponding to an eclipse cannot be done solely by deforming a specific filter stored in the camera in accordance with the above-mentioned conditions.